In the past years, many patents were issued for stirrer fans. With a high frequency heating appliance of the type which has a high frequency oscillator mounted directly on a wall of the heating chamber, it is well known that the high frequency oscillator is disposed at the center of the heating chamber and a stirrer fan is rotated concentrically with a dipole antenna extending from the oscillator in order to ensure good distribution of the heating energy.
A severe limitation is placed on the dimensions of the heating chamber. In other words, in the event that the impedance of the high frequency oscillator, typically a magnetron, doesn't match the impedance of the load, the appliance fails to properly deliver a high frequency output. A conventional approach to attain impedance matching is to adjust the dimensions of the heating chamber. For example, in order to enable the appliance to provide a high frequency output with any size heating chamber, a waveguide is disposed between the heating chamber and the high frequency oscillator for providing an impedance adjustment. This method however entails high frequency loss in the waveguide and thus causes a decrease in the high frequency output.
Another conventional approach is to provide a metal plate fixedly secured in the vicinity of the dipole antenna. However, this method has a problem in that the position of the metal plate varies from appliance to appliance and spark discharges take place thereabout. As another alternative, a metal blade, which is arranged so as to be parallel to the dipole antenna, is mounted on the stirrer fan in the vicinity of the dipole antenna so as to attain impedance matching. Since this approach is inefficient unless the tip of the metal blade is placed close to the wall of the heating chamber bearing the dipole antenna mounted thereon, the approach still has outstanding problems with respect to output power losses, the concentration of the electric field, and an increase in the resistance losses which occur in the metal blade.